29 - Sodium and water homeostasis

Give 3 factors that affect body water distribution:

1) Age

2) Gender

3) Fat content

What percentage of weight does total body water make up in males and females?

- Males (60%)

- Females (55%)

What is the net water flow?

From hypotonic to hypertonic

Describe the distributive effects of water IV:

Has a hydrostatic effect and is evenly distributed in intracellular, extracellular and intravascular compartments

Describe the distributive effects of saline IV:

Has osmotic effect and is distributed in interstitial and vascular fluid

Describe the distributive effects of colloid IV:

Has an oncotic effect and is distributed in intravascular fluid only

What ion does water follow?

Na+

Where is Na+ more concentrated?

Extracellular fluid (ECF)

Where is K+ more concentrated?

Intracellular fluid (ICF)

What is osmolality?

osmoles/kg solvent (mmol/kg)

What is osmolarity?

Osmoles/L of solution (mmol/L)

Can osmolality be measured?

Yes

Can osmolarity be measured?

No must be calculated

How is osmolarity calculated?

2x (Na+K) + urea + glucose (if abnormal) by adding all electrolytes and molecules that have an effect on water movement

How is osmolar gap calculated?

Osmolar gap = measured osmolarity - calculated osmolarity

normal

What does an osmolar gap greater than 10 suggest?

The presence of other osmotically active solutes in the plasma that perhaps should not be there

What is a hypertonic solution?

The solution has a higher solute concentration than the cell so water moves out of the cell and into the solution causing the cell to plasmolyze

What is a hypotonic solution?

The solution has a lower solute concentration than the cell so water moves into the cell causing cells to swell and burst

How do osmoreceptors regulate water balance?

Detect changes in osmolality in ECF

How do barorecptors regulate water balance?

Detect changes in intravascular volume

What are more sensitive to water balance, osmoreceptors or baroreceptors?

Osmoreceptors

When ICF and ECF are equal, what does osmolality equal?

285mmol/Kg

What structure controls the release of ADH?

Hypothalamic osmoreceptors

What structures produce ADH?

Supraoptic and paraventricular nuclei of neurons in the hypothalamus

Where is ADH stored?

Posterior pituitary

What 2 stimuli stimulate ADH release?

1) Shrinkage due to loss of ECF

2) Hypovolaemia

How does ADH increase water reabsorption?

Acts on the collecting duct to increase expression of aquaporins on the membrane

What is the effect of mercury on aquaporin expression?

Inhibits expression

What is the effect of lithium on aquaporin expression?

Decreases expression

What triggers thirst?

Loss of ECF (increases osmolality)

Where does most of the Na+ re absorption take place?

Proximal convoluted tubule

What hormone converts angiotensinogen into angiotensin I?

Renin

Where is renin produced?

Juxtaglomerular cells of the kidney

What coverts angiotensin I to angiotensin II?

ACE

Where is ACE produced?

Lungs

What does angiotensin II stimulate?

Aldosterone secretion from the adrenal cortex

What does aldosterone cause?

Sodium and water retention

What 2 effects do natriuretic peptides cause?

1) Vasodialtion

2) Increased loss of urinary Na+ and water

When are natriuretic peptides released?

When myocytes are mechanically stretched by increased plasma volume

Give 3 examples of natriuretic peptides:

1) Atrial natriuretic peptide (ANP)

2) Brain natriuretic peptide (BNP)

3) C-type natriuretic peptide (CNP)

What is hyponatraemia?

Sodium levels are found below the normal range of 133-146mmol/L

Give 8 symptoms of hyponatraemia: (SALT LOSS)

1) Stupor (coma)

2) Anorexia

3) Lethargy

4) Tendon reflexes decreased

5) Limp muscles

6) Orthostatic hypotension

7) Seizures and headache

8) Stomach cramping

Give 3 diseases which may present hypovolaemia with hyponatraemia?

1) Congestive heart failure

2) Renal disorder

3) Liver cirrhosis

Give 3 causes of Syndrome of Inappropriate ADH (SIADH):

1) Malignant disease

2) Neurological disorders (meningitis)

3) Pulmonary disorders (TB)

Give 3 presentations of SIADH:

1) Hyponatraemia

2) High Na+ in urine

3) Normal renal function

Give 2 treatments used for SIADH:

1) Fluid restriction

2) Demeclocycline (ADH antagonist)

What is hypernatraemia?

Sodium levels found above the normal range of 133-146mmol/L

Give 9 symptoms of hypernatraemia: (FRIED SALT)

1) Flushed skin

2) Restless, anxious

3) Increased blood pressure

4) Edema (peripheral and pitting)

5) Decreased urine output

6) Skin flushed

7) Agitation

8) Low grade fever

9) Thirst

How does Addison's disease affect Na+ levels?

Hypoaldosteronism causes Na+ loss

What is Conn's syndrome?

Primary hyperaldosteronism

How does Conn's disease affect Na+ levels?

Primary hyperaldosteronism causes Na+ excess

What is diabetes insipidus?

A disorder caused by inadequate amounts of ADH which causes excessive water loss

Give 2 general causes of diabetes inspidus:

1) Hypothalamic or pituitary damage

2) ADH receptor or aquaporin complications